This is my nice, clean, reliable Singer 99K, showing the bobbin plate in the open position.
The Singer 99K, 99, 66K and 66 all have the same design of plate. It is designed not to slide open any further than shown in the picture. However it can be a bit of a puzzle if you need to remove the bobbin plate altogether when cleaning the machine.
Over the past few days I have seen the same question asked by a couple of people who were not sure how to remove the bobbin plate. Fortunately there was a mucky 99K in the house, so I was able to make a quick video.
So you can fully appreciate the filth on this machine, here is a photo of the underneath of the bobbin plate, showing how the grooves stop short of the edge so it cannot slide off completely.
And here is a loving close-up of the muck and grease covering the metal runner, each end of which fits into the grooves.